Conventionally, nickel-cadmium cells have been the main cells used, particularly as secondary cells for memory-backups or sources for driving the memory-backups of AV (Audio Visual) and information devices such as personal computers, and VTRs (video tape recorders). Lately, non-aqueous electrolyte secondary cells have been drawing a lot of attention as a replacement for the nickel-cadmium cells because non-aqueous electrolyte secondary cells have advantages of high voltage, high energy density, and displaying excellent self-dischargeability. Various developments of the non-aqueous electrolyte secondary cells have been attempted and a portion of these developments have been commercialized. For example, more than half of notebook type personal computers, cellular phones and the like are driven by the non-aqueous electrolyte secondary cells.
Lithium ion secondary cells using carbon materials that are capable of injecting or removing lithium as active substances for a cathode are the most commercially available materials for the non-aqueous electrolyte secondary cells. Various organic solvents are used for electrolytes of lithium ion secondary cells in order to mitigate the risk when lithium is produced on the surface of cathode, and to increase outputs of driven voltages. For example, electrolytes in which lithium salts are dissolved in mixed solvents of cyclic carbonates and non-cyclic carbonates are widely known (Japanese Patent Application Laid-Open (JP-A) Nos. 2-172162, and 4-171674).
Further, in non-aqueous electrolyte secondary cells for use in cameras, alkali metals (especially, lithium metals or lithium alloys) are used as cathode materials, and electrolytes thereof ordinarily use aprotic organic solvents such as ester organic solvents.
However, although these non-aqueous electrolyte secondary cells are high performance cells, they have a problem with safety as described below.
First, when alkali metals (especially, lithium metals or lithium alloys) are used as cathode materials for non-aqueous electrolyte secondary cells, the alkali metals are extremely highly-active with respect to water. Accordingly, when a non-aqueous electrolyte secondary cell is imperfectly sealed and water enters therein, a problem is caused in that hydrogen is generated due to a reaction of the cathode materials and water, leading to a high risk of ignition or the like.
Moreover, since lithium metals have a low melting point (about 170° C.), they have a problem in that, when a large current suddenly flows into a cell during a short circuit or the like, the cell generates an extreme amount of heat, leading to a high risk of melting the lithium metal of the cell, or the like.
Further, a problem occurs in that the electrolyte comprising the aforementioned organic solvent vaporizes or decomposes due to the heat-generation of the cell, generates gas, and causes an explosion or ignition.
In order to solve the aforementioned problems, for example, a method has been proposed of providing a cylindrical cell with a mechanism for suppressing flow of excessive current, which exceeds a predetermined amount, into the cell by operating a safety valve as well as by breaking an electrode terminal when internal pressure of the cell increases in accordance with the increase of the temperature of the cell during a short circuit or overcharge of the cell (Nikkan Kogyo Shinbun, Electronic Technology, Vol. 39, No. 9, 1997).
However, the mechanism cannot be relied upon to operate normally all the time. In cases in which the mechanism does not operate normally, a problem still remains because it is feared that an amount of heat due to the excessive current will increase, resulting in a risk of ignition or the like.
Accordingly, in order to solve the aforementioned problems, development of a highly safe non-aqueous electrolyte secondary cell, which reduces the aforementioned risks and is not a safety measure providing an attachment part such as a safety valve, is desired.
Performance has been developed to improve performance of the non-aqueous electrolyte secondary cells, and active substances themselves have been studied to improve capacity characteristics thereof. Of the active substances, evolution of carbon materials is noticeable, and of the carbon materials, it has been disclosed that high crystalline carbon materials such as graphite and graphitized carbons are excellent from the viewpoints of having large charging/discharging capacity per unit weight, low average potential during charging/discharging, and large energy density (JP-A Nos. 57-208079 and 5-13088). Low crystalline carbon materials have also been noticed for comprising highly conductive solvents such as ethylene carbonate (EC), γ-butyrolactone (γ-BL) or propylene carbonate (PC) or mixtures thereof with highly dielectric solvents without being limited to the same.
However, it is known that the carbon materials used as active substances for a cathode might have poor compatibility depending upon the type of the electrolytes.
For example, it has been disclosed that propylene carbonate or butylene carbonate i.e., cyclic carbonate decompose at cathodes, and are thereby unsuitable as electrolytes (JP-A Nos. 2-10666 and 4-184872).
It is also known that, when γ-butyrolactone, dimethyl ether (DME) and tetrahydrofuran (THF) are used as electrolytes, a problem is caused in that cathodes and electrolytes react with each other to form coatings on cathode surfaces, increasing interface resistance and deteriorating charging/discharging characteristics at low temperature.
In order to improve the charging/discharging characteristics at low temperature, it has been disclosed that use of electrolytes containing 2-methyltetrahydrofuran (2-MeTHF) as solvent components is effective. It has also been disclosed that electrolytes in which ethylene carbonate (EC) is added to 2-MeTHF tend to show longer charging/discharging duration (U.S. Pat. No. 4,737,424 (1998)).
However, it is known that 2-MeTHF is a combustible solvent which has a low flash point (−11° C.), is volatile and combustible, and is easily oxidized to thereby easily generate explosive peroxides. Accordingly, a problem with safety has been pointed out. With respect to this combustible electrolyte, it is desired to decrease an amount of the electrolyte used and to increase flame retardancy.
In order to prevent decomposition of electrolytes at cathodes, since ethylene carbonate (EC) shows appropriate stability even when used with high crystalline carbon materials, EC is used as an electrolyte having excellent cyclic characteristics. However, since EC has a high freezing point (37° C.) and deteriorates discharging characteristics at low temperature, the EC cannot be used alone. In order to obtain non-aqueous electrolyte secondary cells having excellent discharging characteristics at low temperature, ethylene carbonate (EC) must be mixed with tetrahydrofuran (THF), diethyl carbonate (DEC), dimethyl carbonate (DMC), methylethyl carbonate (MEC) or the like. For example, a three-component mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) or diethyl carbonate (DEC), and ethylmethyl carbonate (EMC) has been proposed (JP-A No. 5-13088).
However, in accordance with a mixing ratio in the three-component mixture described above, ethylene carbonate (EC) is crystallized at low temperature of −20° C., and conductivity decreases, whereby discharging capacity is greatly decreased.
Further, since all of these mixture-type electrolytes are combustible solutions, in the same manner as described above, the problem remains in that the mixture-type electrolytes have an extremely high risk of ignition or the like.